Intel's Core 2 Duo and Core 2 Extreme processors

INTEL'S DESKTOP PROCESSORS HAVE NOT been in a good place for the past two and a half years. Pentium 4 and Pentium D CPUs have run at relatively high clock speeds but delivered relatively low performance compared to their competition from AMD. They've also drawn a tremendous amount of power, which they've generously expended as heat. In other words, they've been hotter than Jessica Simpson and slower than, well, Jessica Simpson. Despite heroic efforts by Intel's engineering and manufacturing types, these chips based on the Netburst micro architecture haven't been able to overcome their inherent limitations well enough to keep up with the Athlon 64. As a result, Intel decided to scrap Netburst and bet the farm on a new high-performance, low-power design from the Israel-based design team responsible for the Pentium M.

The product of that team's efforts is a new CPU micro architecture known as Core, of which the Core 2 Duo and Core 2 Extreme are among the first implementations intended for desktop PCs. We've been knee-deep in hype about the Core architecture for months now, with a stream of juicy technical details, semi-official benchmark previews, and clandestine reviews of pre-release products feeding the anticipation. Clearly, when a player as big as Intel stumbles as badly as it has, PC enthusiasts and most others in the industry are keen to see it get back up and start delivering exciting products once again.

Fortunately, the wait for Core 2 processors is almost over. Intel has decided to take the wraps off final reviews of its new CPUs today, in anticipation of the chips' release to the public in a couple of weeks. Fish have gotta swim, politicians have gotta dissemble, and TR has gotta test hardware, so of course we've had the Core 2 processors on the test bench here in Damage Labs for a thorough workout against AMD's finest—including the new Energy Efficient versions of the Athlon 64 X2. After many hours of testing, we're pleased to report that the Core 2 chips live up to the hype. Intel has recovered its stride, returned to its winning ways, gotten its groove back, and put the izzle back in its shizzle. Read on for our full review.

Conroe up close
We first previewed the chip code-named Conroe back in March, and now we finally have our hands on one within the confines of our own labs. In spite of all the hype, the Core 2 Duo processor itself is a rather unassuming bloke that looks no different than Pentium CPUs that proceeded it. Like them, it resides in an LGA775-style socket and runs on a 1066MHz front-side bus.

Also like its most immediate predecessors, the Core 2 Duo is manufactured on Intel's 65nm fab process. Unlike them, however, the Core 2 Duo is not comprised of two chips crammed together on one package; it's a native dual-core design with a total of roughly 291 million transistors arranged in an area that's 143 mm². By contrast, each of the Pentium Extreme Edition 965's two chips have an estimated 188 million transistors in an 81-mm² die. If you add the two chips together, the Pentium Extreme Edition 965 has more total transistors and a larger total die area than the Core 2 Duo.

Intel plans to offer five flavors of Core 2 processors initially, with prices and features like so:

Model	Clock speed	Bus speed	L2 cache	TDP	Price
Core 2 Extreme X6800	2.93GHz	1066MHz	4MB	75 W	$999
Core 2 Duo E6700	2.67GHz	1066MHz	4MB	65 W	$530
Core 2 Duo E6600	2.4GHz	1066MHz	4MB	65 W	$316
Core 2 Duo E6400	2.13GHz	1066MHz	2MB	65 W	$224
Core 2 Duo E6300	1.86GHz	1066MHz	2MB	65 W	$183

The prices on the mid-range models are quite reasonable once you consider performance, as we'll do shortly. What you'll really want to notice about the Core 2 chips, though, is the column labeled TDP. This parameter—thermal design power—specifies the amount of cooling the chip requires, and the numbers are down dramatically from the Pentium Extreme Edition 965's rating of 130W. Clock speeds are down, as well, since the Core microarchitecture focuses on achieving high performance per clock rather than stratospheric clock frequencies. The fastest Core 2 processor is the X6800 Extreme, which is separated from the regular Core 2 Duos only by its 2.93GHz clock speed and a 10W higher TDP—oh, and by almost half a grand.

Intel says complete PC systems based on the Core 2 Extreme X6800 and individually boxed products will both begin selling on July 27th, while Core 2 Duo processors with 4MB of L2 cache should show up on August 7th. Intel will be transitioning its CPU production gradually away from Pentiums to Core 2 Duos, and that transition might not happen as quickly as the market would like. I wouldn't be surprised to see strong demand and short supply of these processors for the next couple of months, until Intel is able to ramp up production volumes. The less expensive versions of the Core 2 Duo with 2MB of L2 cache are initial casualties of this controlled ramp. They aren't expected to be available until the fourth quarter of this year.

On a brighter note, the supporting infrastructure for Core 2 chips is already fairly well established. The processors should be compatible with a number of chipsets, including the enthusiast-class 975X and the upcoming 965-series mainstream chipsets from Intel. NVIDIA's nForce4 SLI X16 Intel Edition should work, too, as well as the yet-to-be-released nForce 500 series for Intel. In fact, the Core 2 can act as a drop-in replacement for a Pentium D or Pentium Extreme Edition, provided that the motherboard is capable of supplying the lower voltages that Core 2 processors require. Only the most recent motherboards seem to have Core 2 support, so you'll want to check carefully with the motherboard maker before assuming a board is compatible. Our Core 2 Duo and Extreme review samples, for example, came from Intel with an updated version of the D975XBX motherboard, since older revisions couldn't supply the proper voltage.
Speaking of which, the upgrade path for those who buy motherboards for Core 2 processors in the next few months isn't entirely clear. The server/workstation version of the Core microarchitecture, the Woodcrest Xeon, already rides on a faster 1333MHz front-side bus. The Core 2 Duo may move to this faster bus frequency at some point, but Intel hasn't revealed a schedule for this move. Intel has revealed plans to deliver "Kentsfield," a quad-core processor with two Conroe chips in a single package, in early 2007, but we don't yet know whether current motherboards will be able to support it. Investing in a Core 2-capable motherboard right now might be a recipe for longevity, but it might also be a dead end as far as CPU upgrades are concerned.
What's with the name?
Before we go on, we should probably take a moment to talk about the Core 2 Duo product name. It's dreadful, of course, but for deeper reasons than you might think. You see, microprocessors tend to be known by several names throughout their lives, and usually those names aren't really related. For example, the chip code-named Willamette, based on a microarchitecture called Netburst, became the first product known as Pentium 4. The multiple names may be a little difficult to keep straight, but they're distinctive and follow a coherent logic.
This chip, however, is different. The microarchitecture is called Core, the chip is code-named Conroe, and the product is called Core 2 Duo. By that logic, the chip code-named Willamette would have been based on the Willette microarchitecture, and the first product might have been the Willette 4 Quadro, which everyone knows is actually a disposable razor.

The Core 2 Duo's name does make sense from a certain perspective, though, because Intel has been shipping the original Core Duo as a dual-core mobile processor since the beginning of the year. There's also a single-core version of that processor known as the Core Solo, which explains the whole Duo suffix. And the mobile version of the Core 2 Duo, based on the chip code-named Merom, will be the follow-up to the Core Duo.
So why name the microarchitecture Core? You've got me. The Core microarchitecture is a descendant of the one found in the current Core Duo, but it's been pretty extensively reworked and certainly deserves a new name. The fact that its name matches up with the previous-gen product's name is confounding. We'll simply have to, as one Intel employee admonished at the Spring '06 IDF, "Deal with it."
The Core microarchitecture
The heritage of the Core microarchitecture can be traced back through the Core Duo and Pentium M, through the Pentium II and III, all the way to the original Pentium Pro. That original design has undergone some serious evolutionary changes, plus a few radical mutations, along the way, and the Core microarchitecture may be the most sweeping set of changes yet. Even compared to its direct forebear, the Core Duo, the Core design can be considered substantially new.
Core's genesis was a project known internally at Intel as Merom, whose mission was to build a replacement for the Pentium M and Core Duo mobile processors. The Israel-based design team responsible for Intel's mobile CPUs followed a distinctive design philosophy focused intently on energy efficiency, which helped make the Pentium M a resounding success as part of the Centrino platform. When power and heat became problems for Netburst-based desktop and server processors, Intel turned to Merom as the source of a new, common microarchitecture for its mobile, desktop, and server CPUs
Because of its orientation toward power efficiency, the Core architecture is a very different design from Netburst. From the very first Pentium 4, Netburst was a "speed demon" type of architecture, a chip designed not for clock-for-clock performance, but to be comfortable running at high clock frequencies. To this end, the original Netburst processors had a relatively long 20-stage main pipeline. For a time, this design achieved good results at the 130nm process node, but all of that changed when Intel introduced a vastly reworked Netburst at 90nm. With its pipeline stretched to 31 stages and its transistor count up significantly, the Pentium 4 "Prescott" still had trouble delivering high clock speeds without getting too hot, and performance suffered as a result.
The Core architecture, meanwhile, is the opposite of a speed demon; it's a "brainiac" instead. Core has a relatively short 14-stage pipeline, but it's very "wide," with ample execution resources aimed at handling lots of instructions at once. Core is unique among x86-compatible processors in its ability to fetch, decode, issue and retire up to four instructions in a single clock cycle. Core can even execute 128-bit SSE instructions in a single clock cycle, rather than the two cycles required by previous architectures. In order to keep all of its out-of-order execution resources occupied, Core has deeper buffers and more slots for instructions in flight.

Like other contemporary PC processors, Core translates x86 instructions into a different set of instructions that its internal, RISC-like core can execute. Intel calls these internal instructions micro-ops. Core inherits the Pentium M and Core Duo's ability to fuse certain micro-op pairs and send them down the pipeline for execution together, a provision that can make the CPU's execution resources seem even wider that they are. To this ability, Core adds the capability to fuse some pairs of x86 "macro-ops," such as compare and jump, that tend to occur together commonly. Not only can these provisions enhance performance, but they can also reduce the amount of energy expended in order to execute an instruction sequence.

Another innovation in Core is a feature Intel has somewhat cryptically named memory disambiguation. Most modern CPUs speculatively execute instructions out of order and then reorder them later to create the illusion of sequential execution. Memory disambiguation extends out-of-order principles to the memory system, allowing for loads to be moved ahead of stores in certain situations. That may sound like risky business, but that's where the disambiguation comes in. The memory system uses an algorithm to predict which loads are to move ahead of stores, removing the ambiguity.

This optimization can pay big performance dividends.

In contrast to the various "dual-core" implementations of Netburst, the Core microarchitecture is a natively dual-core design. The chip's two execution cores each have their own separate, 32K L1 instruction and data caches, but they share a common L2 cache that can be either 2MB or 4MB in size. (The execution trace cache from Netburst is not carried over here.) The chip can allocate space in this L2 cache dynamically on an as-needed basis, dedicating more space to one core than the other in periods of asymmetrical activity. The common cache also eliminates the need for coherency protocol traffic on the system's front-side bus, and one core can pass data to another simply by transferring ownership of that data in the cache. This arrangement is easily superior to the Pentium D's approach, where the two cores can communicate and share data only via the front-side bus.

As Intel's brand-new common microarchitecture, Core is of course equipped with all of the latest features. String 'em together, and you get something like this: MMX, SSE, SSE2, SSE3, SSE4, EM64T, EIST, C1E, XD, and VT, to name a subset of the complete list. The most notable addition here is probably EM64T—Intel's name for x86-64 compatibility—because the Core Duo didn't have it. In order to make its way into desktops and servers, Core needed to be a 64-bit capable processor, and so it is.

The scope and depth of the changes to the Core microarchitecture simply from its direct "Yonah" Core Duo ancestor are too much to cover in a review like this one, but hopefully you have a sense of things. For further reading on the details of the Core architecture, let me recommend David Kanter's excellent overview of the design.

AMD answers with Energy Efficient Athlons
Anticipating better power efficiency from Intel's new desktop processors, AMD has begun offering Energy Efficient versions of many of its CPUs for the new Socket AM2 infrastructure. Much like the Turion 64 mobile processor and the HE versions of the Opteron server chips, these Energy Efficient Athlon 64s have been manufactured using a tweaked fabrication process intended to produce chips capable of operating at lower voltages. Making these more efficient chips isn't easy, so AMD charges a price premium for the Energy Efficient models that averages about 40 bucks over the non-EE versions.

Just as we wrapped up our testing of the Core 2 Duo, a pair of these new Energy Efficient processors arrived from AMD. On the right above is the EE version of the Athlon 64 X2 4600+. AMD rates its max thermal power at 65 W, down from 89W in the stock version. Currently, the X2 4600+ EE commands a $43 price premium over the regular X2 4600+.

The processor on the left above may have the longest product name of any desktop CPU ever: "Athlon 64 X2 3800+ Energy Efficient Small Form Factor." This long-winded name, though, signals a very frugal personality; AMD rates this processor's max thermal power at only 35W. Making the leap from the stock version to the EE SFF model will set you back roughly 60 bucks, or you can stop halfway and get the X2 3800+ EE with a 65W TDP for 20 bucks more than the basic 89W version.

By the way, you may be tempted to compare the TDP numbers for the Core 2 Duo with these processors, but there is some risk in doing so. AMD generates its TDP ratings using a simple maximum value, while Intel uses a more complex method that produces numbers that may be less than the processor's actual peak power use. As a result, direct comparisons between AMD and Intel TDP numbers may not reflect the realities involved.

For all intents and purposes beyond power consumption and the related heat production, the EE versions of the Athlon 64 X2 ought to be identical to the originals. They run at the same clock speeds, have the same feature sets, and should deliver equivalent performance. Because that's so, and due to limited testing time, we've restricted our testing of these Energy Efficient chips to power consumption.

Our testing methods
Please note that the two Pentium D 900-series processors in our test are actually a Pentium Extreme Edition 965 chip that's been set to the appropriate core and bus speeds and had Hyper-Threading disabled in order to simulate the actual products. Similarly, our Socket AM2 versions of the Athlon 64 X2 4800+, 4600+, and 4200+ are actually the Athlon 64 FX-62 and X2 5000+ clocked down to the appropriate speeds, and the Core 2 Duo E6600 is actually an underclocked Core 2 Extreme X6800. The performance of our "simulated" processor models should be identical to the actual products.
Also, I've placed asterisks next to the memory clock speeds of the Socket AM2 test systems in the table below. Due to limitations in AMD's memory clocking scheme, a couple of these systems couldn't set their memory clocks to exactly 800MHz.

As ever, we did our best to deliver clean benchmark numbers. Tests were run at least three times, and the results were averaged.